Measurement of the flow field characteristics in single and dual S-shape 90° bends using matched refractive index PIV

•The flowfield in single and dual S-bends was measured using PIV.•The refractive index of the fluid was matched to the acrylic test sections.•The turbulence kinetic energy on the mid plane was higher in the second bend of the S-bend.•Mean secondary motions are present on the inner wall side at the start of the first bend.•The mean secondary motions shift toward the outer wall as the flow moves around the second bend.

The flow field in single and dual S-shape short radius 90° bends was measured using matched refractive index Particle Image Velocimetry (PIV) at Reynolds numbers of 40,000 and 70,000. The high velocity region in the first bend of the S-bend moves closer to the outer wall as it moves along the curvature than in the single bend. The flow then accelerates further along the inner wall of the second bend. The averaged in-plane components of the turbulence kinetic energy in the single bend and the first bend of the S-bend are similar, with an increase along the outer wall and downstream of the separation zone on the bend inner wall. The turbulence kinetic energy in the second bend was much higher than in the first bend of the S-bend. Counter rotating vortices develop in the first bend and propagates into the second bend. High velocity secondary flows are observed near the second bend inner wall at approximately 40° along the second bend.